Hi Alexis,you can take lessons in any of the 5star Hotels as they all have coaches.If this is not as per your budget you could go to contact the Nepal lawn Tennis Association at Sato Bato, located near the Ringroad (from jawalakhel direction Airport) or at Ranga Sala, Tripureshwor,pass Bluebird Supermarket, direction World Trade Centre, at the chowk turn sharp right. At Sato Bato you can contact Surya Bhusan Bajracharya, at Ranga Sala contact Milan.
Regards
Gerlinde
PS: I have tennis rackets for sale and quite a lot of tennis cloth - if you are interested please look at my list, posted 26th of March.